1. Field of the Invention
The present invention relates to magnetic powder and an isotropic bonded magnet. More particularly, the present invention relates to magnetic powder and an isotropic bonded magnet which is produced, for example, using the magnetic powder.
2. Description of the Prior Art
For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetization of the magnetic powder and the content of the magnetic powder contained in the bonded magnet. Accordingly, when the magnetization of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.
At present, most of practically used high performance rare-earth bonded magnets use the isotropic bonded magnets which are made using MQP-B powder manufactured by MQI Inc. as the rare-earth magnetic powder thereof. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets represented by those manufactured using the MQP-B powder involve the following problems.
(1) The conventional isotropic bonded magnets do not have a sufficiently high magnetic flux density. Namely, because the magnetic powder that has been being used has poor magnetization, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.
(2) Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable when they are practically used in motors are very low. Further, these magnets have poor heat stability due to their small coercive forces.
(3) The conventional bonded magnets have low corrosion resistance and heat resistance. Namely, in these magnets, it is necessary to increase the content of the magnetic powder to be contained in the bonded magnet in order to compensate the low magnetic properties (magnetic performance) of the magnetic powder. This means that the density of the bonded magnet becomes extremely high. As a result, the corrosion resistance and heat resistance of the bonded magnet are deteriorated, thus resulting in low reliability.
It is therefore an object of the present invention to provide magnetic powder that can produce a bonded magnet having excellent magnetization and having excellent reliability especially excellent temperature characteristics (that is, heat resistance and heat stability), and provide an isotropic bonded magnet formed from the magnetic powder.
In order to achieve the above object, the present invention is directed to magnetic powder composed of an alloy composition represented by Rx(Fe1xe2x88x92yCoy)100xe2x88x92xxe2x88x92zxe2x88x92wBzAlw (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.02-1.5 at %), the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein the magnetic powder has magnetic properties characterized in that:
when the magnetic powder is formed into an isotropic bonded magnet having a density xcfx81[Mg/m3] by mixing with a binding resin and then molding it, the remanent magnetic flux density Br[T] at the room temperature satisfies the relationship represented by the formula of Br/xcfx81[x10xe2x88x926 Txc2x7m3/g]xe2x89xa70.125;
the irreversible susceptibility ("khgr"irr) of the isotropic bonded magnet which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of xe2x88x923.8xc3x9710xe2x88x926 H/m as a starting point is equal to or less than 5.0xc3x9710xe2x88x927 H/m; and
the intrinsic coercive force (HCJ) of the isotropic bonded magnet at the room temperature is in the range of 320-720 kA/m.
According to the magnetic powder as described above, it is possible to provide bonded magnets having excellent magnetic properties as well as excellent heat resistance (heat stability) and corrosion resistance.
In the present invention, it is preferred that when the magnetic powder is formed into an isotropic bonded magnet by mixing with a binding resin and then molding it, the absolute value of the irreversible flux loss (initial flux loss) is equal to or less than 6.2%.
This makes it possible to provide bonded magnets having especially excellent heat resistance (heat stability).
In these cases, it is preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. This makes it possible to improve saturation magnetization of the hard phase of the composite structure (in particular, nanocomposite structure), and thereby the coercive force is further enhanced.
Further, it is also preferred that said R includes Pr and its ratio with respect to the total mass of said R is 5-75%. This makes it possible to improve the coercive force and rectangularity with less drop of the remanent magnetic flux density.
Further, it is also preferred that said R includes Dy and its ratio with respect to the total mass of said R is equal to or less than 14%. This makes it possible to improve the coercive force and the heat resistance (heat stability) without markedly lowering the remanent magnetic flux density.
In the present invention, it is also preferred that the magnetic powder is obtained by quenching the alloy of a molten state. According to this, it is possible to refine the microstructure (crystalline grains) relatively easily, thereby enabling to further improve the magnetic properties of the bonded magnet.
Further, it is also preferred that the magnetic powder is obtained by milling a melt spun ribbon of the alloy which is manufactured by using a cooling roll. According to this, it is possible to refine the microstructure (crystalline grains) relatively easily, thereby enabling to further improve the magnetic properties of the bonded magnet.
Furthermore, it is also preferred that the magnetic powder is subjected to a heat treatment for at least once during the manufacturing process or after its manufacture. According to this, homogeneity (uniformity) of the structure can be obtained and influence of stress introduced by the milling process can be removed, thereby enabling to further improve the magnetic properties of the bonded magnet.
In the magnetic powders described above, it is preferred that the average particle size lies in the range of 0.5-150 xcexcm. This makes it possible to further improve the magnetic properties Further, when the magnetic powder is used in manufacturing bonded magnets, it is possible to obtain bonded magnets having a high content of the magnetic powder and having excellent magnetic properties.
Another aspect of the present invention is directed to an isotropic bonded magnet formed by binding a magnetic powder containing Al with a binding resin, wherein the isotropic bonded magnet is characterized in that:
when the density of the isotropic bonded magnet is defined as xcfx81[Mg/m3], the remanent magnetic flux density Br[T] at the room temperature satisfies the relationship represented by the formula of Br/xcfx81[x10xe2x88x926 Txc2x7m3/g]xe2x89xa70.125;
the irreversible susceptibility ("khgr"irr) of the isotropic bonded magnet which is measured by using an intersect ioning point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of xe2x88x923.8xc3x9710xe2x88x926 H/m as a starting point is less than 5.0xc3x9710xe2x88x927 H/m; and
the intrinsic coercive force (HCJ) of the isotropic bonded magnet at the room temperature is in the range of 320-720 kA/m.
According to the magnetic powder as described above, it is possible to provide an isotropic bonded magnet having excellent magnetic properties as well as excellent heat resistance (heat stability) and corrosion resistance.
In this isotropic bonded magnet, it is preferred that said magnetic powder is formed of R-TM-Bxe2x80x94Al based alloy (where R is at least one rare-earth element and TM is a transition metal containing Iron as a major component thereof). This also makes it possible to provide an isotropic bonded magnet having particularly excellent magnetic properties as well as particularly excellent heat resistance (heat stability) and corrosion resistance.
Further, in this isotropic bonded magnet, it is also preferred that the magnetic powder is composed of an alloy composition represented by Rx(Fe1xe2x88x92yCoy)100xe2x88x92xxe2x88x92zxe2x88x92wBzAlw (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.02-1.5 at %). This also makes it possible to provide an isotropic bonded magnet having particularly excellent magnetic properties as well as particularly excellent heat resistance (heat stability) and corrosion resistance.
Moreover, in this isotropic bonded magnet, it is also preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. This makes it possible to improve the coercive force of the bonded magnet.
In this case, it is preferred that said R includes Pr and its ratio with respect to the total mass of said R is 5-75%. This makes it possible to improve the coercive force and rectangularity with less drop of the remanent magnetic flux density.
Further, it is also preferred that said R includes Dy and its ratio with respect to the total mass of said R is equal to or less than 14%. This makes it possible to improve the coercive force and the heat resistance (heat stability) without markedly lowering the remanent magnetic flux density.
In the isotropic bonded magnets as described above, it is preferred that the average particle size of the magnetic powder lies in the range of 0.5-150 xcexcm. This makes it possible to obtain an isotropic bonded magnet having a high content of the magnetic powder and having excellent magnetic properties.
Further, in the isotropic bonded magnets as described above, it is also preferred that the absolute value of the irreversible flux loss (initial flux loss) is equal to or less than 6.2%. This makes it possible to provide particularly excellent heat resistance (heat stability).
Furthermore, in the isotropic bonded magnets as described above, it is also preferred that the magnetic powder is constituted from a composite structure having a soft magnetic phase and a hard magnetic phase. This improves magnetizability and heat resistance (heat stability), thus leading to less deterioration in the magnetic properties with elapse of time.
Preferably, the isotropic bonded magnets as described above are to be subjected to multipolar magnetization or have already been subjected to multipolar magnetization. According to this, satisfactory magnetization can be made even in the case where sufficient magnetizing magnetic field is not obtained, thereby enabling to obtain sufficient magnetic flux density.
Further, preferably, the isotropic bonded magnets as described above are used for a motor. By using the bonded magnet to motors, it becomes possible to provide small and high performance motors.